System and method for digital memorized predistortion for wireless communication

ABSTRACT

A power amplifier system includes an input operable to receive an original value that reflects information to be communicated and an address data former operable to generate a digital lookup table key. The power amplifier system also includes a predistortion lookup table coupled to the address data former and a power amplifier having an output and coupled to the predistortion lookup table. The power amplifier system further includes a feedback loop providing a signal associated with the output of the power amplifier to the predistortion lookup table and a switch disposed in the feedback loop and operable to disconnect the predistortion lookup table from the output of the power amplifier.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/619,538, filed on Sep. 14, 2012, which is a continuation of U.S.patent application Ser. No. 11/262,079, now U.S. Pat. No. 8,326,238,filed on Oct. 27, 2005, which is a continuation of U.S. patentapplication Ser. No. 10/137,556, filed on May 1, 2002, now U.S. Pat. No.6,985,704. Each of these references is hereby incorporated by referencein its entirety for all purposes.

BACKGROUND OF THE INVENTION

The present invention relates to wireless communication. The presentinvention is especially applicable to mobile wireless communication andprocessing of signals to compensate for nonlinearities associated withpower amplifiers used for wireless transmission.

Next-generation wireless communication will utilize improved transmittertechnology for variety of broadband and multimedia services, supportedby advanced potable equipment and handsets. For a long time, the longerhandset battery life-time, better call quality and less wirelesshigh-frequency radiation to human brain are three most concerning andinsolvable issues for mobile phone designers and users. Actually, theabove three problems are closely related to spectral efficiency andpower efficiency of handset, because both higher spectral efficiency andpower efficiency will greatly improve mobile system performance, extendhandset battery life-time and reduce handset transmitted power to someextent. However, the nonlinear distortion and low DC conversionefficiency introduced by power amplifier (PA) in current handset RFtransmitter impact severely performance of wireless system and shortensgreatly battery life-time of handset.

Next-generation wireless communication will require improved transmittertechnology for variety of broadband and multimedia applications,supported by advanced improved base stations and access points topotable equipment and handsets. The spectral efficiency and powerefficiency are among the most important requirements of mobilecommunication systems. For many years, the designers of wirelesscommunication system have been concerning the issue of PA linearizationin RF transmitter, because it is closely related to the development ofhighly spectral efficiency modulation scheme. It has been demonstratedthat the spectrally efficient linear modulation technologies such asQPSK and QAM have high spectral efficiency under the case of linearamplification. However, in order to obtain the highest power efficiency,the nonlinear power amplifier such as Class AB, C or D is required infinal amplifying stage of RF transmitter. Unfortunately, the high powerefficiency of nonlinear amplifiers generates nonlinear inter-modulationproducts in adjacent channels, which results in both amplitude toamplitude (AM-AM) and amplitude to phase (AM-PM) distortion. Thenonlinear distortion will cause spectral broadening and high out-of-bandpower emission of output signal. As the result, the signal spectrumexpands into adjacent channels to produce interference for other users.Furthermore, these inter-modulation distortion products are spaced soclose to the desired signal and cannot simply be filtered out byconventional filters. In order to avoid the nonlinear distortion, asimple and usual solution is to back off output signal from thesaturation range of PA so that signal level is confined to the poweramplifier. However, this will result in a less power efficient operationbecause several decibels of back off are required usually to obtainappropriate linearity. Obviously, the approach is not suitable to theadvanced wireless system design that should be a high capacity andefficient digital transmission system.

For a long time, the longer handset battery's life-time, less radioradiation to human brain and high voice quality have been the mostconcerning problems by handset designers, makers and users. Actually,the key to solve above problems is closely related to develop thehandset RF system with spectral efficiency and power efficiency. Thehigher spectral efficiency and power efficiency not only improve greatlymobile system performance, but also extend battery life-time of handsetand reduce transmitter's radiation which will be helpful to protectusers' brain from radiation to some extent.

Current predistortion technologies used widely to linearize PA in mobilecommunication system are mainly analog predistorter implemented at IF/RFby means of analog circuit and digital predistorter at baseband withdigital signal processing (DSP) technique.

The analog predistorter is based on the principle of error subtractionand power match to realize linearization of PA, and, hence must use anauxiliary PA to match the main PA. Under a perfect matching, the errorof auxiliary PA will compensate nonlinear distortion caused by main PA.Because nonlinear feature of PA is very complicated and many variablesare involved, the analog predistortion has only less predistortionaccuracy and consumes more power.

In contrast, the DSP-based predistorter is usually preferred since ithas stable characteristics that perform in a wide range of temperatures,and eliminates the necessary of tuning in factory. Therefore, it isbetter suitable to the fast tracking and adjusting any possible changesin PA parameters, such as drifts due to temperature, aging and operatingpoint variations.

Unfortunately, although DSP-based predistortion technologies haveadvantages over analog ones, it is difficult for current digitalpredistortion schemes to apply to handset and wideband system because ofcomplexity in hardware organization 25 and DSP algorithm.

SUMMARY OF THE INVENTION

In order to avoid interference and enhance performance of handset, RFpower amplifier of handset should have a near-linear conversioncharacteristic. The linearization characteristics can be obtained, forexample, by systems and methods for predistortion according toembodiments of the present invention.

According to one embodiment of the present invention, there is a systemfor signal processing in preparation for wireless transmission, thewireless transmission being from a portable wireless communicationdevice, wherein the wireless transmission includes use of a poweramplifier having nonlinear characteristics. The system comprises: memoryfor storing digitally-indexed information, wherein the digitally-indexedinformation models nonlinear characteristics of the power amplifier, andthe digitally-indexed information is stored prior to processing of afirst signal that reflects information to be communicated; first logic,configured to accept the first signal and to retrieve, based on thefirst signal, a portion of the digitally-indexed information stored inthe memory; second logic, configured to generate a second signal basedon the portion of the digitally-accessed information and on the firstsignal, wherein the second signal pre-compensates for the nonlinearcharacteristics of the power amplifier, and the second signal is forwireless transmission based on the second signal.

According to another embodiment of the present invention, there is aportable electronic device having wireless communication capabilities.The portable electronic device comprises: a processor for executingcommands that direct operations of the portable electronic device; awireless transmission stage that includes a power amplifier havingnonlinear characteristics; memory for storing digitally-indexed data,including digitally-indexed information that reflects nonlinearcharacteristics of the power amplifier; and code stored in the memorythat directs the processor to: determine, given a first signal thatincludes information to be communicated wirelessly, a correctioncontribution based on the signal and on the digitally-indexedinformation that reflects nonlinear characteristics of the poweramplifier; and pre-compensate for the nonlinear characteristics of thepower amplifier, based on the correction contribution, to obtain asecond signal, wherein the wireless transmission stage will transmitwirelessly based on the second signal.

According to another embodiment of the present invention, there is, in aportable mobile device, a method for processing signals in preparationfor wireless transmission, wherein the wireless transmission includesuse of a power amplifier. The method comprises: maintaining a lookuptable that contains pre-computed distortion contributions, thedistortion contributions being for distorting of signals to therebypre-compensate the signals for nonlinear characteristics of the poweramplifier; accepting a value that reflects information to becommunicated, the value hereinafter referred to as original value;generating a lookup-table key based on the original value, wherein thelookup-table key is digital; retrieving from the lookup table, using thelookup-table key, a pre-computed distortion contribution for theoriginal value; distorting the original value based on the pre-computeddistortion contribution to obtain a distorted value to pre-compensatefor the nonlinear characteristics of the power amplifier; and wirelesslytransmitting based on the distorted value.

These and other embodiments of the present invention are further madeapparent, in the remainder of the present document, to those of ordinaryskill in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more fully describe embodiments of the present invention,reference is made to the accompanying drawings. These drawings are notto be considered limitations in the scope of the invention, but aremerely illustrative.

FIGS. 1 and 2 show typical AM-AM and AM-PM behaviors of PA to bepredistorted.

FIG. 3 shows a training schematic diagram of digital adaptivepredistorter to linearize power amplifier in handset, which is anembodiment of the present invention.

FIG. 4 shows schematically a structure of adaptive predistorter inhandset when the training finished, which is an embodiment of thepresent invention.

FIG. 5 shows schematically the lookup tables' arrangement in theadaptive digital predistortion scheme.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

The description above and below and the drawings of the present documentfocus on one or more currently preferred embodiments of the presentinvention and also describe some exemplary optional features and/oralternative embodiments. The description and drawings are for thepurpose of illustration and not limitation. Those of ordinary skill inthe art would recognize variations, modifications, and alternatives.Such variations, modifications, and alternatives are also within thescope of the present invention. Section titles are terse and are forconvenience only.

Preferred embodiments of the present invention relate to a novel digitaladaptive predistorter to linearize power amplifier (PA) in RFtransmitter of mobile stations, including variety of mobile potableequipment, handsets and PDA, for CDMA, TDMA, 20 GSM, GPRS, 3G (UMTS,W-CDMA, CDMA2000, 3GPP and others), WLAN system that transmits thecomplex modulated signal with aid of quadrature modulator and poweramplifier. Because power amplifier in RF transmitter distorts RF outputsignal, the digital predistorter is used to correct non-linearity of PAby predistortion in opposite sense to PA input. The preferred circuitarrangement in embodiments of the present invention is speciallydesigned for all wireless mobile stations or handsets, and also can beused in base stations or access points and other wireless communicationsystems such as microwave and satellite communications. Preferredembodiments of the present invention present a practicable DSP-basedpredistortion algorithm and organization using to handset withoutincreasing manufacture cost but improving greatly handset performance.

In accordance with the preferred embodiment architecture, a presenteddigital adaptive predistorter is designed to linearize wireless RFtransmitter for all handsets system, such as for CDMA, TDMA, GSM, 3G andWLAN system, and the like. For the application of predistortiontechnology in handset, the following issues are especially of interest:

-   1. Dynamic predistortion control range to track and correct    non-linearity of PA in wider range;-   2. Fast convergence speed to swiftly track characteristics of PA;-   3. Requirement for low power consumption to extend handset battery    life-time, which needs time-saving software algorithm and less    complicity hardware structure;-   4. Being able to use the existing source and chip room in handset    for manufacture cost consideration.

Based on above considerations, a new design of adaptive memorizedpredistorter 15 for wireless handsets, according to an embodiment of thepresent invention has the following properties:

-   1. Using a stored compensation principle's structure to make the    predistorter be able to memory great amount non-linear    characteristics of PA for much better predistortion performance in    wider dynamic range;-   2. Using a time-delay adaptive structure to improve non-linearity    tracking;-   3. Using a simple architecture in implementation to use the existing    DSP room in handsets without additional hardware circuit for low    power consumption and low manufacture cost. 25

The components of embodiment architecture shown in FIG. 3 are describedas follows:

-   00: Coder to generate the required modulation data under wireless    system specifications-   10 I-Q: Address data formers to obtain the required address data-   11 I-Q: Base band symbol wave shaping filters-   12 I-Q: N-bit vector multipliers to result the predistorted signals    for I-Q channel respectively-   13 I-Q: Digit to analog converters-   14 I-Q: Analog reconstruction filters-   15: Quadrature modulator-   16: Nonlinear power amplifier-   17: Antenna-   20 I-Q: Address registers-   21 I-Q: Memory tables to store linearity of PA and time-delay    characteristics of channels-   22 I-Q: Square circuits to generate the envelope of reference    signals v_(d)-   23 I-Q: Envelope comparator to generate the error signals e_(p)-   24 I-Q: Switch T/C: T is in the training status when system is    trained and C is in the calling status when system finishes training-   25 I-Q: Square circuits of feedback channels to generate the    envelope of feedback signals v_(f)-   26 I-Q: Analog to digit converters-   27 I-Q: Analog receiving filters-   28: Demodulator-   29 Local oscillator to generate a high frequency carrier signal-   30 I-Q: Address registers of lookup tables-   31 I-Q: Predistortion lookup tables to store nonlinear    characteristic of PA-   32 I-Q: Adders of lookup tables-   33 I-Q: Step size factors of adaptive algorithm-   34 I-Q: Switch On/Off: On is when system is trained, and Off is when    the training finished

Power Amplifier (PA) Model

Prior to further describing the predistorter according to embodimentsthe present patent, the characteristic of PA is discussed initially.

Power amplifier is the final stage of handset RF transmitter, andnormally consumes the most electrical power. For the high efficientlinear modulations such as QPSK or QAM, a linear high power amplifiermust be used to maintain optimum spectral efficiency and low out-of-bandemission. Such a linear amplifier, however, does not usually have a goodDC to RF power conversion efficiency. For instance, traditional class-Apower amplifier has a maximum theoretical power conversion efficiency of50%. This is a major drawback for mobile application, especially for theportable battery operated equipment such as handsets, where battery lifeis of major importance. The power conversion efficiency can be obtainedby using nonlinear power amplifier, such as class AB, C or D poweramplifier. However, these amplifiers distort the input-output signal andcause spectral broadening and high out-of-band power emission of theoutput signal.

The PA in mobile communication system should typically be operated closeto saturation or even saturated so as to maximize power efficiency. Thesaturation has serious repercussions on the signal to be amplified, andexhibits nonlinear characteristics such as amplitude and phasedistortion that lead to an undesirable inter-modulation interference inthe neighboring frequency band. The amplitude and phase characteristicsused in this patent are shown in FIGS. 1 and 2, respectively.

Adaptive Predistorter

FIG. 3 shows the organization of handset digital predistorter from baseband through RF part. The predistorter consists of address data formers10I-10Q, linearity and time-delay lookup tables 21I-21Q, predistortionlookup tables 31I-31Q, vector multipliers 12I-12Q, error comparators23I-23Q, and two types of switches 24I-24Q and 34I-34Q. All signals inthe architecture are denoted by v(t) with the corresponding subscriptsto express their location in system.

In accordance with a preferred implementation of the presentedinvention, the coded 1/Q data symbols to be transmitted are fed intoboth digital base band waveshaping filters 11I-11Q and address dataformer 10I-10Q, respectively.

The address data formers 10I-10Q are designed to generate the requiredbinary signal format. The data formers 10I-10Q receive signal from coder00 first, and then transform the received signal to the sign symbol withform either 0 or 1. The binary data are used as the address of bothpredistortion lookup tables 31I-31Q and linearity and time-delay lookuptables 21I-21Q.

The linearity and time-delay tables 31I-31Q are used to store thelinearity characteristics of PA 16 and time-delay parameters ofchannels. The signal time-delay of channel is caused when I- andQ-signal from the output of shaping filters 11I-11Qpass through thecircuit components, such as multiplier 12I-12Q, DAC 13I-13Q,reconstruction filters 14I-14Q, QM 15, PA 16, DQM 28, receiver filters27I-27Q, ADCs 26I-26Q, squire circuits 25I-25Q and switchers 24I-24Q tothe comparers 23I-23Q. The 20 stored signals in the tables 21I-21Q arefirst obtained by adaptive iteration, and then are used as linearreference model to compare feedback signal that contains nonlineardistortion component. The lookup tables 21I-21Q are updated by adaptivealgorithm in training period. When the training period is over, therequired linear and time-delay information are stored in the tables forpredistortion tables' training. As the result, there is no necessary tobuild a special time-delay circuit for channel time-delay estimate ofhandset.

The predistorter lookup tables 30I-30Q are used to store thepredistortion signal to linearize power amplifier. The requiredpredistorted signals in the tables are obtained by adaptive training andby comparing the outputs of tables 21I-21Q and feedback signals. Theoutputs v_(r)(k) of lookup table 30I-30Q are fed to multipliers 12I-12Q,and multiply with signals v_(m)(k) from shaping filters 11I-11Q toresult a predistorted signal v_(p)(k) that is an inverse non-linearityof PA to predistort the input of PA.

The switch ON/OFF controllers 34I-34Q are set to ON status whenpredistortion lookup tables 31I-31Q are trained by adaptive algorithm.After the training procedure completed, the switches are set to Offstatus and the predistortion lookup tables 34I-34Q are no longer updatedadaptively.

The switch T/C controllers 24I-24Q are used to choose handset circuitstatus. When the lookup tables are trained, the switches are set toTraining status so that two types of lookup tables 21I-21Q and 31I-31Qare updated by adaptive algorithm. When the training processing iscompleted, the controllers are switched to the calling status and alllookup tables in handset stop being updated.

Handset is trained by two phases. First one is to obtain and store thelinearity of PA and time-delay characteristics of channels to thelinearity and time-delay tables. Second one is to generate the requiredpredistortion signals and to store them into predistortion lookuptables. The training time for two phases will take about 0.5˜1 second.Preferably, the training time for two phases is less than about 4seconds. When all adaptive training finished, the handset predistortioncan be implemented by the organization shown as FIG. 4. This is a verysimple structure and may use the existing DSP room in handset forprogramming without adding any extra hardware circuit, and therefore isa cost-saving and high efficiency approach.

Predistortion Lookup Tables

The predistortion lookup tables for I- and Q-channel store thenon-linearity of PA in inverse to AM-AM and AM-PM distortion to correctnonlinear distortion of RF transmitter. The predistortion lookup tablescan be configured in a number of ways. However, a drawback of lookuptable structure in traditional predistorter is memory less system, slowtracking speed and complicity structure, and therefore is unable to beused in handsets.

The new memorized predistorter structure presented in this patentintroduces memory function into predistortion scheme by addressingoperation. When the training procedure completed, the predistortionlookup tables just need to read out the signal stored in table entriesaccording to the corresponding address for distortion correction. Thus,the predistortion of handset is completed by a simple and fast approachwithout great amount calculating for each correcting sampler so as toreduce of power consumption of handset.

The lookup tables of predistorter are based on a stored compensationprinciple that maps a set of input data into a digital output, and areupdated adaptively by a time delay algorithm. The output signal oflookup table is actually related to the previous N transmitted data, andtherefore has a memory function when compensate the nonlinearity of PA.

Linearity and Time-Delay Tables

The linearity and time-delay tables for I- and Q-channel are used tostore the linearity of PA and time-delay characteristics of channel byadaptive training processing. In order to obtain linearity of PA, twolarger sizes of lookup tables should be designed for I- and Q-channel,respectively, to obtain the linear reference signal with time-delay. Thetables' outputs are compared with the feedback signals to result in theweighted error signals with nonlinear distortion for predistortiontables updating by a adaptive algorithm. After the training procedurefinishes, the linearity and time-delay tables accomplish their mission.

Address of Lookup Table

The address of lookup table in predistorter is formed by the followingapproach. At first, the signal complex envelope of PA output can beexpressed as

v _(m) =v _(Q) G(|v _(Q)|²)=H(v _(p))G(|H(v _(p))|²)  (1)

where V_(Q) is the output of quadrature modulator, v_(p) is thepredistorted signal, H is an assumed transform function from DAC toquadrature modulator, and G is a level-dependent complex gain of poweramplifier. We see from predistorter architecture of FIG. 3 that thecomplex gain predistorter is described by the following complex gainequation

v _(p) =v _(m) F(V _(m))=v _(m) v ₁  (2)

where F is the mapping function of lookup table, which maps aN-dimensional vector V_(m) to real output. In fact, the N-dimensionalvector represents a set of N-bit address of lookup table, expressed by

V _(m)=(sg ₁ ,sg ₂ , . . . ,sg _(N))^(T)  (3)

where each binary symbol sg_(i) in above vector can be obtained by thefollowing expression

$\begin{matrix}{{sg}_{i} = \left\{ \begin{matrix}1 & {{v_{m} = 1},} & {1 \leq i \leq N} \\0 & {{v_{m} = {- 1}},} & {1 \leq i \leq N}\end{matrix} \right.} & (4)\end{matrix}$

which transforms the data from coder to the binary sign with the form of0 or 1.

Based on mobile system architecture, the two predistortion tables areused for I and Q-channel, respectively, to map any possible combinationsof input binary symbol to the lookup table output, written as

v _(rl) =F _(l)(V _(ml)) v _(rQ) =F _(Q)(V _(mQ))

where v _(rl) =Re(v _(r)), v _(rQ) =lm(v _(r)) and V _(ml) =Re(V _(m)) V_(mQ) =lm(V _(m)).  (5)

Adaptive Methodology for Predistortion Lookup Table

The adaptive memorized lookup table consists of address register andmemory table. The address of lookup tables are formed in a shiftregister by first taking binary symbol by means of Eq. (3) and (4) andthen sequentially feeding the binary symbol into the shift register. Theaddress determines the corresponding entry of lookup table that storesinformation related to previous N transmitted data to correctnon-linearity of PA.

Assume that the address of lookup table is formed by N-bits shiftregister, then the lookup table contains M=2^(N) entries. Clearly, eachoutput of lookup table is a function of last N transmitted data. Thus,there exist M possible estimates for PA nonlinearity at any compensationinstant, and only one of them is selected as the output of lookup tableby means of the address formed by N transmitted signal. The outputsignal is used to compensate and correct nonlinear distortion of P A.

The lookup table structure, based on the memory compensation principle,is involved only very simple logical operation and less complexityhardware structure, therefore, is better suitable to handset and higherbit rate wide band wireless communication system. We denote the addressvector of table at the kth time as

V _(m)(k)=(sg ₁(k),sg ₂(k), . . . ,sg _(N)(k))^(T)  (6)

All possible input vectors from k to previous k−M+1 time are expressedby the following set

A={V _(m)(k),V _(m)(k−1), . . . ,V _(m)(k−M+1)}  7)

which records M possible estimates of PA nonlinearity, denoted by

R(k)=(v _(r)(k),v _(r)(k−1), . . . ,v _(r)(k−M+1))^(T)  (8)

At the same time, only one of the estimates is read out from the lookuptable

v _(r)(k)=r _(q(k))(k) q(k)=V _(m)(k)εAε{0,1}^(N)  (9)

where the subscript q(k)=V_(m)(k) denotes the address determined by Ninput binary symbol characters at kth time. Thus, the mapping function Fof lookup table can be written by

F(V _(m)(k))=r _(q(k))(k) V _(m)(k)ε{0,1}N,rεR  (10)

Note that F is unknown and hardly expressed mathematically beforeadaptive procedure beginning. However, F may be determined adaptively byupdating lookup table entries under the adaptive algorithm to realizeall possible mapping that corresponds to the relations with {0,1}^(N)→R.

The entries of lookup table can be updated by the following iteration

$\begin{matrix}{{r_{i}\left( {k + 1} \right)} = \left\{ \begin{matrix}{r_{i}(k)} & {{1 \leq i \leq M},{i \neq {q(k)}}} \\{{r_{i}(k)} + {\mu \; {e_{p}(k)}}} & {i = {q(k)}}\end{matrix} \right.} & (11)\end{matrix}$

where e_(p)(k) is the error signal, and μ is the step size ranged from0<μ<1 to control the convergence rate and steady-state of algorithm.

The error signal e_(p)(k) contains both AM-AM and AM-PM components of PAso that the memorized predistorter could track adaptively variation ofamplitude and phase characteristics of PA. The envelope error using toupdate lookup table entries is given by

e _(p)(k)=v _(d) ²(k)−v _(f) ²(k)  (12)

where v_(f) ²(k) and v_(d) ²(k) express the envelopes of feedback signalv_(f)(k) and reference signal v_(d)(k) respectively.

Adaptive Updating of Linearity and Time-Delay Lookup Table

The adaptive training for linearity and time-delay table is based on thetraditional MSE updating algorithm. The entries of lookup table areupdated by the following iteration

$\begin{matrix}{{r_{i}\left( {k + 1} \right)} = \left\{ \begin{matrix}{r_{i}(k)} & {{1 \leq i \leq M},{i \neq {q(k)}}} \\{{r_{i}(k)} + {\mu \; {e_{i}(k)}}} & {i = {q(k)}}\end{matrix} \right.} & (13)\end{matrix}$

where the error signal e, (k) is given by

e _(f)(k)=v _(d)(k)−v _(f)(k)  (14)

where v_(f)(k) and v_(d)(k) express the feedback signal with linearcharacteristic and reference signal that is the output of table,respectively.

(Other) Observations

Embodiments of the present invention may be adapted for use for allwireless systems regardless the modulation types (such as QAM, QPSK,OFDM and others) and PA models used in wireless systems includingvariety of mobile stations, handsets, base stations and access pointssuch as, for example:

-   -   Current wireless system: CDMA, TDMA, GSM, GPRS and their        extension systems;    -   Next generation broadband wireless system: CDMA2000, UMTS,        WCDMA, 3GPP, WLAN (802.11 a, b specifications) and their        extension systems;    -   PDA and potable mobile PC for WLAN (802.11 a, b) system and        their extension systems.

In an embodiment, an arrangement of lookup table address introduces astored compensation function into predistorter. As the result, thepredistorter is of memory function when it estimates and correctsnonlinear distortion of RF transmitter, which is extremely effective toprovide a dynamic predistortion correction of AM-AM and AM-PM distortionin wider range.

In an embodiment, an arrangement the stored compensation functiondepends on the address data in lookup table. Therefore, eachpredistortion output of lookup table is not only related to the currentinput data but also related to last N transmitted data. Actually, theoutput of predistorter is a function of last N transmitted symbol.

In an embodiment, the predistortion correction for AM-AM and AM-PMdistortion can be implemented by a measure of vector multiplicationbetween the outputs of lookup tables and the output shaping filters.Also, other approach such as vector adding can be used to implement thepredistortion correction. Because all entries of lookup table areadaptively updated by a weighted envelope error that contains both AM-AMand AM-PM characteristics of PA, the outputs of lookup tables containthe inverse AM-AM and AM-PM distortion components.

In an embodiment, the predistorter uses two types of lookup tables fordifferent roles in handset predistortion. One is the predistortion tableto provide predistortion signal for the non-linearity correction of PA.Another is the linearity and time-delay table to store the linearity ofPA and time-delay characteristics of channel, which will be used asreference signal for the training of predistortion table.

In an embodiment, the linearity of PA and time-delay characteristics ofchannel are obtained by adaptive training After the algorithm converges,the memorized information in the table is the reference signal requiredby predistortion table. Because the reference signal also contains thetime-delay characteristics of channel of handset, there is no necessaryto build a special time-delay circuit for estimating the time-delay ofchannel for the power consumption saving in handset.

In an embodiment, the linearity and time-delay table is trained byadaptive algorithm when PA of handset is set in the linear region sothat the linear characteristics of PA could be pick up and stored intothe entries of table. After algorithm converges, the required linear andtime-delay characteristics of handset are stored in the table, and PA isset back to its non-linear operating region working as Class C or Damplifier.

In an embodiment, the linearity and time-delay table will be trainedfirst to obtain the linear and time-delay characteristics of handset.Then predistortion table is trained by the reference signal fromlinearity and time-delay table to obtain the required predistortionsignal and be stored in predistortion table for nonlinear distortioncorrection.

In an embodiment, the training for both linearity table and thepredistortion table ill take around 0.5 to 1 second. Afterward, thelinearity and time-delay table accomplished its mission and becomesstatic table without any output and being updated. Also thepredistortion table is no longer updated by adaptive algorithm, and onlyresponds to the input data in address register to output thecorresponding predistortion signal. Thus, in an embodiment, the lookuptables in the predistorter will not be longer be updated after thetraining. For example, they will not be updated for at least onetelephone call. For example, they will not be updated for at least oneday of use. An actual handset predistortion structure is very simple andmay use the existing DSP chip in handset for all predistortion signalprocessing.

Throughout the description and drawings, example embodiments are givenwith reference to specific configurations. It will be appreciated bythose of ordinary skill in the art that the present invention can beembodied in other specific forms. Those of ordinary skill in the artwould be able to practice such other embodiments without undueexperimentation. The scope of the present invention, for the purpose ofthe present patent document, is not limited merely to the specificexample embodiments of the foregoing description, but rather isindicated by the appended claims. All changes that come within themeaning and range of equivalents within the claims are intended to beconsidered as being embraced within the spirit and scope of the claims.

What is claimed is:
 1. A method of operating a power amplifier, themethod comprising: initializing the power amplifier; performing atraining phase comprising: establishing pre-computed distortioncontributions based on pre-compensation training feedback signalsrepresentative of output of the power amplifier; and storing thepre-computed distortion contributions in a lookup table; and performingan operating phase comprising: switching a controller off to disconnectsignal representative of the output of the power amplifier; accepting anoriginal value that reflects information to be communicated; generatinga digital lookup table key based on the original value; retrieving fromthe lookup table, using the digital lookup table key, a correspondingpre-computed distortion contribution for the original value; distortingthe original value based on the corresponding pre-computed distortioncontribution to obtain a distorted value to pre-compensate for thenonlinear characteristics of the power amplifier; and wirelesslytransmitting a pre-distorted signal based on the distorted value.
 2. Themethod of claim 1 wherein the distorted value pre-compensates for thenonlinear characteristics of the power amplifier.
 3. The method of claim1 wherein establishing pre-computed distortion contributions comprises:establishing reference information by training based on referencetraining feedback signals via feedback channels, the reference trainingfeedback signals being representative of the output of the poweramplifier; and storing the reference information in a reference table.4. The method of claim 3 wherein the reference information compriseslinearity characteristics of the power amplifier and time-delayparameters of the feedback channels.
 5. The method of claim 3 whereinestablishing each respective pre-computed distortion contributioncomprises: squaring an in-phase component of a respectivepre-compensation training feedback signal to produce a squared in-phaseoutput value; squaring a quadrature component of the respectivepre-compensation training feedback signal to produce a squaredquadrature output value; squaring an in-phase component of acorresponding reference signal according to the reference information toproduce a squared in-phase reference value; squaring a quadraturecomponent of the corresponding reference signal according to thereference information to produce a squared quadrature reference value;determining an in-phase difference between the squared in-phasereference value and the squared in-phase output value; determining aquadrature difference between the squared quadrature reference value andthe squared quadrature output value; and determining the respectivepre-computed distortion contribution based on the in-phase differenceand the quadrature difference.
 6. The method of claim 5 wherein thereference information and the pre-computed distortion contributions areestablished within four seconds.
 7. The method of claim 1 wherein theoperating phase endures for at least one entire telephone conversation.8. The method of claim 1 wherein the power amplifier is a component of amobile device.
 9. The method of claim 1 wherein the power amplifier is acomponent of a base station.
 10. A method of operating a power amplifierof a mobile handset, the method comprising: powering on the mobilehandset; populating a lookup table with pre-computed distortioncontributions; switching off a controller coupled to feedback signalsassociated with the power amplifier; accepting an original value thatreflects information to be communicated; generating a digital lookuptable key based on the original value; retrieving from the lookup table,using the digital lookup table key, a corresponding pre-computeddistortion contribution for the original value; distorting the originalvalue based on the corresponding pre-computed distortion contribution toobtain a distorted value to pre-compensate for the nonlinearcharacteristics of the power amplifier; and wirelessly transmitting apre-distorted signal based on the distorted value.
 11. The method ofclaim 10 wherein populating the lookup table comprises: establishing thepre-computed distortion contributions by training based onpre-compensation training feedback signals representative of output ofthe power amplifier; and storing the pre-computed distortioncontributions in the lookup table.
 12. The method of claim 11 whereinestablishing the pre-computed distortion contributions comprises:establishing reference information by training based on referencetraining feedback signals via feedback channels, the reference trainingfeedback signals being representative of the output of the poweramplifier; and storing the reference information in a reference table,wherein the reference information comprises linearity characteristics ofthe power amplifier and time-delay parameters of the feedback channels.13. The method of claim 10 wherein the controller remains in the offcondition for at least one entire communication session.
 14. The methodof claim 10 wherein the controller remains in the off condition whilethe mobile handset is powered on.
 15. The method of claim 10 wherein atime delay between turning on the mobile handset and switching off thecontroller is less than four seconds.
 16. A power amplifier systemcomprising: an input operable to receive an original value that reflectsinformation to be communicated; an address data former operable togenerate a digital lookup table key; a predistortion lookup tablecoupled to the address data former; a power amplifier having an outputand coupled to the predistortion lookup table; a feedback loop providinga signal associated with the output of the power amplifier to thepredistortion lookup table; and a switch disposed in the feedback loopand operable to disconnect the predistortion lookup table from theoutput of the power amplifier.
 17. The power amplifier system of claim16 wherein the power amplifier system is a component of a mobile device.18. The power amplifier system of claim 16 wherein the power amplifiersystem is a component of a base station.
 19. The power amplifier systemof claim 16 wherein the feedback loop couples the power amplifier andthe predistortion lookup table during a training phase and the switchdecouples the power amplifier and the predistortion lookup table duringa communication phase.
 20. The power amplifier system of claim 16further comprising an antenna coupled to the output of the poweramplifier.